Process for producing bimodal polyethylene resins

ABSTRACT

A process for producing bimodal polyethylene resins in two reactors in series, the process comprising producing a first polyethylene resin fraction in a first slurry loop reactor in a diluent in the presence of a catalyst and producing a second polyethylene resin fraction in a second slurry loop reactor, serially connected to the first reactor, in the diluent in the presence of the catalyst, the first polyethylene resin fraction being passed from the first reactor to the second reactor together with the catalyst, one of the first and second reactors producing a resin fraction of higher molecular than the resin fraction produced by the other of the first and second reactors, characterised in that the first reactor is fed with a feed of ethylene and diluent having an ethylene content of at least 70 wt % based on the weight of the diluent and in that in the first reactor the slurry of polyethylene in the diluent has a solids content of at least 30 wt % based on the weight of the diluent

[0001] The present invention relates to a process for producing bimodalpolyethylene resins, in particular such resins for use as pipe resins oras film or blow moulding resins. Most particularly, the presentinvention relates to a process for producing bimodal polyethylene resinsin two reactors in series.

[0002] It is known to produce polyethylene resins for use as pipe orfilm or blow moulding resins in two reactors in series in a liquid fullslurry loop process. One of the reactors produces a high molecularweight fraction and the other of the reactors produces a low molecularweight fraction. The resultant bimodal resin, comprising a chemicalblend of the two fractions has particular application, for example, foruse as a pipe resin which exhibits good mechanical properties such asenvironmental stress crack resistance (ESCR) and slow crack growthresistance. It is known that in order to improve the mechanicalproperties of the resin, it is desired to separate as far as possiblethe polymerisation reactions occurring in the two reactors so that thedifference in density and molecular weight between the two resinfractions is enhanced for any given target density of the resin. While anumber of processes are known in the art for enhancing the separation ofthe polymerisation reactions occurring in the two reactors, neverthelessthere is still a need in the art for an improved polymerisation processfor the production of bimodal polyethylene pipe resins which stillfurther increases the reactor independence. Some of these processes aredisclosed for example in the following prior art documents:

[0003] EP-A-649,860 discloses of process for co-polymerising ethylene intwo liquid full loop reactors serially connected, wherein the comonomeris introduced in the first reactor, wherein the high molecular weightfraction of the polymer is produced in the first reactor and the lowmolecular weight fraction is produced in the second reactor and whereinthe transfer from the first to the second reactor is operated throughone or more settling legs.

[0004] EP-A-580,930 discloses a process for homo- or co-polymerisingethylene in two liquid full loop reactors serially connected, whereinthe comonomer is introduced in the first reactor and wherein theconcentration of hydrogen is very low in the first reactor and very highin the second reactor.

[0005] EP-A-897,934 discloses a process for preparing ethylene polymersin two serially connected reactors wherein an ethylene homopolymerhaving a melt flow index MI2 of 5 to 1000 g/10 min is formed in thefirst reactor and a copolymer of ethylene and hexene having a melt flowindex MI5 of 0.01 to 2 g/10 min is formed in the second reactor.

[0006] EP-A-832,905 discloses a process for preparing ethylene homo- orco-polymers in two liquid full loop reactors serially connected, in thepresence of a chromium-based catalyst, wherein a low molecular weighthomopolymer of ethylene is produced in the first reactor and a highmolecular weight homo- or co-polymer of ethylene is produced in thesecond reactor.

[0007] WO 92/12181 discloses a method for homo- or co-polymerisingethene in the presence of a Ziegler-Natta catalyst and a possiblecomonomer and hydrogen. The polymerisation is carried out in a loopreactors at a temperature higher than the critical temperature but lowerthan the melting temperature of ethene, and at a pressure higher thanthe critical pressure of the mixture.

[0008] In single slurry loop reactors for the production of monomodalpolyethylene resins, it is known that if the pump which is provided inthe loop reactor for circulating the slurry around the loop is providedwith high output power, this can lead to higher solids concentration inthe slurry circulating around the reactor. Furthermore, the solidsconcentration can also be increased by using a circulating pump having adiameter that is larger than the diameter of the reactor tube. This isachieved by providing a localised enlargement in the reactor tube at thelocation of the propeller or vanes of the pump. Furthermore, it is knownthat the replacement of the conventional settling legs in a slurry loopreactor, which are provided for periodic and sequential take off of thepolyethylene fluff by a so-called “continuous product take off” can alsolead to higher solids concentration in the reactor.

[0009] The present invention provides a process for producing bimodalpolyethylene resins in two reactors in series, the process comprisingproducing a first polyethylene resin fraction in a first slurry loopreactor in a diluent in the presence of a catalyst and producing asecond polyethylene resin fraction in a second slurry loop reactor,serially connected to the first reactor, in the diluent in the presenceof the catalyst, the first polyethylene resin fraction being passed fromthe first reactor to the second reactor together with the catalyst, oneof the first and second reactors producing a resin fraction of highermolecular than the resin fraction produced by the other of the first andsecond reactors, characterised in that the first reactor is fed with afeed of ethylene and diluent having an ethylene content of at least 70wt % based on the weight of the diluent and in that in the first reactorthe slurry of polyethylene in the diluent has a solids content of atleast 30 wt % based on the weight of the diluent.

[0010] In the preferred aspect of the invention, the catalyst is aZiegler-Natta catalyst and the slurry in the first reactor has a solidscontent of from 30 to 60 wt % based on the weight of the diluent.Optionally, the catalyst is a Ziegler-Natta catalyst and the feed ofethylene and diluent for the first reactor has an ethylene content offrom 70 to 250 wt % based on the weight of the diluent.

[0011] In an alternative preferred aspect of the invention, the catalystis a metallocene catalyst and the slurry in the first reactor has asolids content of from 35 to 60 wt % based on the weight of the diluent.In accordance with this preferred aspect, optionally the feed ofethylene and diluent for the first reactor has an ethylene content offrom 90 to 250 wt % based on the weight of the diluent.

[0012] More preferably, the slurry of polyethylene and diluent in thefirst reactor has a solids content of at least 45 wt % and the firstpolyethylene resin fraction is continuously removed from the firstreactor.

[0013] Preferably, the relatively high molecular weight fraction ispolymerised in the first reactor by copolymerisation of ethylene and acomonomer comprising at least one α-olefin having from 3 to 12 carbonatoms. The preferred comonomers are butene, hexene and octene, the mostpreferred one being 1-hexene. Preferably, the comonomer concentration isfrom 2 to 15 wt % based on the weight of the diluent.

[0014] Preferably, the relatively low molecular weight polyethyleneresin fraction is produced in the second reactor by homopolymerisationof ethylene in the presence of hydrogen. Preferably, the hydrogen ispresent in the second reactor in an amount of from almost 0 to 5 mole%based on the weight of the diluent in the second reactor. Near to 0 vol%occurs for example with metallocene catalysts that consume the majorpart of the hydrogen fed to the reactor.

[0015] The present invention is predicated on the discovery by thepresent inventor that when bimodal polyethylene resins are produced intwo reactors in series, the utilisation of increased solids content inthe first reactor by achieving a minimum proportion of ethylene monomerfeed to diluent feed in the first reactor, irrespective of whether thefirst reactor produces the relatively high molecular weight fraction orthe relatively low molecular weight fraction of the ultimate bimodalpolyethylene resin blend, this can lead to enhanced reactor independencebetween the first and second reactors. This consequently tends toenhance the properties of the polyethylene resin blend, particularlywhen the blend is for use as a pipe resin. Most particularly, the slowcrack growth resistance, as determined by a full notch creep test (FNCT)or a notched pipe test (NPT) is greatly enhanced by the process of thepresent invention. In addition, the resin produced in accordance withthe invention has improved resistance to rapid crack propagation asmeasured by the Charpy impact strength test. Furthermore, the use ofhigher solids content and increased ethylene feed to the first reactoralso increases the catalyst productivity.

[0016] Embodiments of the present invention will now be described by wayof example only with reference to the accompanying drawings, in which:

[0017]FIG. 1 is a schematic diagram of an apparatus comprising twoserially connected slurry loop reactors for use in an embodiment of theprocess for producing bimodal polyethylene resins in accordance with thepresent invention;

[0018]FIG. 2 is an enlarged schematic view of part of a settling leg ofthe first slurry loop reactor of the apparatus of FIG. 1;

[0019]FIG. 3 is a graph showing schematically the relationship betweenthe amount of polyethylene fluff recovered from a region of the settlingleg shown in FIG. 2 with the solids content of the slurry in the firstreactor;

[0020]FIG. 4 is a graph showing schematically the relationship betweenthe C2/diluent feed ratio for the reactor without settling and thesolids content in the reactor;

[0021]FIG. 5 is a graph showing schematically the relationship betweenthe C2/diluent for the reactor at 100% of settling and the fluff bulkdensity in the reactor; this is close to the maximum C2/diluent feedratio for a given bulk density;

[0022]FIG. 6 is a graph showing schematically the relationship betweenthe maximum solids content in the reactor and the fluff bulk density inthe reactor; and

[0023]FIG. 7 is a graph showing schematically the relationship betweenthe slow crack growth resistance as determined by the notched pipe test(NPT) and the density of the polyethylene fluff in the first reactor.

[0024] Referring to FIG. 1, there is shown schematically an apparatusfor producing bimodal polyethylene resins in accordance with theinvention in two reactors in series. The apparatus, designed generallyas 2, comprises a first slurry loop reactor 4 and a second slurry loopreactor 6 serially connected thereto and downstream thereof. The firstslurry loop reactor 4 is provided with a feed, indicated as feed No. 1,along an inlet line 8. Feed No. 1 comprises ethylene monomer, one of (a)a comonomer comprising at least one α-olefin having from 3 to 12 carbonatoms or (b) hydrogen, and a catalyst. The catalyst may comprise aZiegler-Natta catalyst, a chromium-based catalyst, in particularcomprising chromium oxide deposited on a support such as silica, ametallocene catalyst, or a LTM (Late Transition Metal) catalyst. Theslurry loop reactor 4 is of generally known construction and is providedwith a settling leg 10 which depends downwardly from the first slurryloop reactor 4. In this embodiment the settling leg 10 is located at theupper portion 9. At the bottom of the settling leg 10 is provided avalve 12 for selective and periodic removal of polyethylene fluff fromthe settling leg 10. In large scale reactors, there is a series of legs(like 10) and corresponding valves (like 12) operating successively. Forcontinuous product take off, valve 12 remains open. An outlet line 14communicates between the valve 12 and the second slurry loop reactor 6for feeding from the first slurry loop reactor 4 to the second slurryloop reactor 6 the first polyethylene resin fraction produced in thefirst slurry loop reactor together with the catalyst. The first slurryloop reactor 4 is additionally provided with a pump 16 which includes apropeller 18 driven by a motor 20. The propeller 18 is located in alower portion 22 of the first slurry loop reactor 4 and the lowerportion 22 is provided with an increased internal diameter (D) comparedto the internal diameter (d) of the remaining parts of the first slurryloop reactor 4.

[0025] The second slurry loop reactor 6 is also provided with arespective inlet line 24 through which a feed (feed No. 2) of additionalethylene monomer and one of (a) a comonomer comprising an α-olefinhaving from 3 to 12 carbon atoms or (b) hydrogen are fed to the secondslurry loop reactor 6. The second slurry loop reactor 6 is also providedwith a respective settling leg 26 and associated valve 28, in thisembodiment located at an upper portion 29 of the second slurry loopreactor 6. An outlet line 30 is provided at the downstream end of thevalve 28 for feeding the ultimate bimodal polyethylene resin, comprisinga blend of the first and second polyethylene resin fractionsrespectively produced in first and second reactors 4,6, for furtherprocessing, for example by an extruder (not shown).

[0026] The second slurry loop reactor 6 is additionally provided with arespective pump 32 comprising a propeller 34 driven by a motor 36.

[0027] In one aspect of the process of the invention, a relatively highmolecular weight polyethylene resin fraction is produced in the firstslurry loop reactor 4 by copolymerisation of ethylene and the comonomerand a relatively low molecular weight polyethylene resin fraction isproduced in the second slurry loop reactor 6 by homopolymerisation ofethylene in the presence of hydrogen. In an alternative aspect of theinvention the low molecular weight resin fraction is produced in thefirst slurry loop reactor 4 and the high molecular weight fraction isproduced in the second slurry loop reactor 6. For either aspect, it hasbeen found that the use of a high solids concentration in the first loopreactor leads to improved mechanical properties of the resultant bimodalpolyethylene resin blend.

[0028] In accordance with the process of the invention, the slurrycirculated around the first slurry loop reactor 4 by the pump 16 has asolids content of at least 30 wt % based on the weight of the diluent,more preferably from 30 to 60 wt % for a Ziegler-Natta catalyst or from35 to 60 wt % for a metallocene catalyst. Most preferably, for eithercatalyst, the solids content of the slurry in the first slurry loopreactor 4 is at least 40 wt % or more preferably at least 45wt % basedon the weight of the diluent.

[0029] The higher solids content in the first loop reactor is achievedby providing that the feed for the first slurry loop reactor 4 has anethylene content of at least 70 wt % based on the weight of the diluent,more preferably from 85 to 250 wt %, yet more preferably above 90 wt %for Ziegler-Natta catalysts and preferably from 90 to 250 wt %, morepreferably above 110 wt %, based on the weight of the diluent, for ametallocene catalyst.

[0030] The present invention can be employed to produce not only piperesins but also film resins and blow moulding resins exhibiting improvedmechanical properties, in particular improved slow crack growthresistance.

[0031] For example, the present invention can produce PE100 pipe resinswith a 50/50 wt % concentration of low molecular weight and highmolecular weight fractions. The high molecular weight fraction may havea density of less than 0.930 g/cc, most preferably less than 0.926 g/cc,with in particular the density being less than 0.928 g/cc formetallocene catalysts, and most particularly less than 0.925 g/cc formetallocene catalysts. The high molecular weight fraction maycorrespondingly have a high load melt index (HLMI) measured inaccordance with the procedures of ASTM D 1238 at a load of 21.6 kg and atemperature of 190° C. of less than 1 g/10 min, more preferably lessthan 0.5 g/10 min, with the corresponding values of less than 2.5 g/10min for metallocene catalysts, more preferably less than 1 g/10 min formetallocene catalysts. For the low molecular weight fraction, thedensity is preferably higher than 0.958 g/cc, typically higher than0.950 g/cc for metallocene catalysts, and most preferably the lowmolecular weight fraction is pure homopolymer. The low molecular weightfraction preferably has a melt index MI₂ measured in accordance with theprocedures of ASTM D 1238 at a load of 2.16 kg and a temperature of 190°C. of at least 20 g/10 min, more preferably above 35 g/10 min.

[0032] For example, the present invention can produce bimodal MDPE filmresins with a 50/50 wt % concentration of low molecular weight and highmolecular weight fractions. The high molecular weight fraction may havea density of less than 0.920 g/cc, most preferably less than 0.917 g/cc.The high molecular weight fraction may correspondingly have a high loadmelt index (HLMI) of less than 5 g/10 min, more preferably less than 2.5g/10 min. For the low molecular weight fraction, the density preferablyranges from 0.940 g/cc to pure homopolymer, typically from 0.950 g/cc topure homopolymer. The low molecular weight fraction preferably has amelt index MI₂ higher than 10 g/10 min, more preferably higher than 24g/10 min.

[0033] All these ranges depends on the weight fraction of high molecularweight in the final product. The ones given here are for a 50/50proportion between the high molecular weight and the low molecularweight fractions. For example, reducing the high molecular weightfraction decreases the density of the high molecular weight resin atconstant final resin density. Typically, for pipe resins the blendcomprises from 35 to 65 wt % high molecular weight fraction, morepreferably from 40 to 57 wt % high molecular weight fraction. For filmresins, the range is broader depending on the key resin properties thatare required. Typically, the high molecular weight fraction comprisesfrom 30 to 75 wt % of the blend.

[0034] In the first slurry loop reactor 4, the first polyethylene resinfraction is polymerised and progressively the polyethylene resin fluffsettles in the settling leg 10. An enlarged schematic view of thesettling leg 10 is illustrated in FIG. 2. It may be seen that thepolyethylene resin fluff 38 has progressively settled at the bottom ofthe settling leg 10. The valve 12 is ordinarily closed. When it isdesired to transfer the first polyethylene resin fraction 38 from thefirst slurry loop reactor 4 to the second slurry loop reactor 6,together with the catalyst therein, the valve 12 is opened and a volumeV, as shown in FIG. 2, of both the polyethylene resin fluff 38 and aminor proportion of slurry 40 is transferred along outlet line 14 to thesecond slurry loop reactor 6.

[0035] In order to enhance reactor independence between the first andsecond slurry loop reactors 4,6, it is desired that the proportion ofslurry 40 in the volume V is minimized. This not only ensures a maximumefficiency in the transfer of the first polyethylene resin fraction 38from the first reactor 4 to the second reactor 6, but also ensures thatthe transfer of comonomer or hydrogen, depending on, respectively,whether or not the high molecular weight or low molecular weightfraction is polymerised in the first slurry loop reactor 4, istransferred additionally in the volume V to the second slurry loopreactor 6. Desirably, the amount of comonomer or hydrogen transfer tothe second slurry loop reactor is minimized in order to ensure reactorindependence, leading not only to a high difference in density betweenthe first and second polyethylene resin fractions, but also a largedifference in the molecular weight. Ideally, the high molecularweight/low density polyethylene resin fraction should be as low aspossible in density and high as possible in molecular weight. Equally,the low molecular weight/high density polyethylene resin fraction shouldbe as high as possible in density and as low as possible in molecularweight.

[0036] For a process configuration where the low molecular weightfraction is produced in the second reactor, it is relatively easy toachieve a large difference in molecular weight, because an extremely lowhydrogen feed in the first reactor and a high hydrogen feed only to thesecond reactor gives very different molecular weight products; as wellknown in Ziegler Natta and metallocene catalysts, the hydrogen is a veryeffective chain termination agent. The achievement of increased reactorindependence tends to increase the density difference between the tworesin fractions. For the opposite process configuration, where the highmolecular weight fraction is produced in the second reactor, hydrogentends to be transferred with the slurry from the first reactor into thesecond reactor. However, without being bound to the theory it isbelieved that the production of the low molecular weight in the firstreactor with metallocene or LTM or hydrogen scavenging catalysts and thehigh molecular weight in the first reactor for Ziegler Natta catalystsare preferred configurations.

[0037] Higher reactor independence therefore allows the enlargement ofthe difference in molecular weight between the two fractions. For anygiven bimodal pipe resin, this comprises a proportion of the highmolecular weight fraction and a proportion of the low molecular weightfraction. The proportions of the two fractions can of course be varied.However, for a 50/50 wt % blend between the two fractions, it is clearthat in order to achieve a target density of the combined blend formingthe pipe resin, which target density is required commercially, for anydecrease in density of the high density, low molecular weight fractionthere must be a corresponding increase in the density of the low densityhigh molecular weight fraction. Increased reactor independence enablesthe achievement of enlarged density differences or enlarged differencesin molecular weight between the two fractions, leading to improvedmechanical properties.

[0038] Referring to FIG. 3, it may be seen that the amount of thepolyethylene fluff in the region V shown in FIG. 2 tends to increasewith an increase in the solids content of the slurry in the first slurryloop reactor. It may thus be seen that an increase of the solids contentof the slurry circulated around the first slurry loop reactor enhancesthe proportion of polyethylene fluff transferred from the settling legof the first reactor 4 to the second reactor 6, which in turn reducesthe amount of comonomer or hydrogen transferred to the second reactor 6,which again in turn increases reactor independence.

[0039] Both the maximum solids content of the slurry and the maximumamount of ethylene in the diluent are dependent on the choice ofcatalyst, for example a Ziegler-Natta catalyst or metallocene catalyst.This is because the nature of the polyethylene fluff, in particular thebulk density of the polyethylene fluff, is catalyst dependent. Thechoice of catalyst affects the morphology of the polyethylene resinfluff and the solids content of the slurry is limited to a maximum valueat which formation of agglomerates by the polyethylene fluff occurs,which inhibits circulation of slurry around the loop in conjunction withreliable settling in the settling leg, which agglomeration in turndepends on the morphology of the fluff and the bulk density. The bulkdensity of the PE fluff ranges from about 0.3 to 0.5 g/cc. When using aZiegler-Natta catalyst, the bulk density of the polyethylene resin istypically about 0.3 g/cc and when using a metallocene catalyst thepolyethylene resin fluff typically has a higher bulk density of about0.4 g/cc. This can also be the case with prepolymerized catalysts. It isdesired to achieve the maximum solids content in the slurry whichenables the settling of polyethylene fluff in the settling leg to bemaximised, but with a minimum amount of the ethylene/diluent ratio whichin turn is dependent upon the achievable bulk density, which is catalystdependent.

[0040] Thus when using a Ziegler-Natta catalyst for example, the bulkdensity is relatively low and so the maximum solids content is achievedby utilising a maximum ethylene/diluent weight ratio in the firstreactor of about 1.2. In contrast, when using a metallocene catalyst themaximum bulk density of the polyethylene fluff is higher, up to about0.5 g/cc, and this permits a higher maximum ethylene/diluent ratio ofabout 2.5 to be employed. Around 92 to 99.5% of the ethylene feed intothe first loop reactor is polymerised to polyethylene and so theethylene/diluent ratio is selected for achieving the maximum solidscontent to achieve maximum settling, but with the corresponding minimumproportion of ethylene in the diluent which is dependent upon theachievable bulk density based on the catalyst employed.

[0041] The polymerisation temperature in the first loop reactor 4typically ranges from 70 to 100° C., most preferably about 80° C. whenthe HMW fraction is produced in the first reactor and from 80 to 120°C., most preferably around 95° C. when the LMW is produced in the firstreactor. The polymerisation temperature also affects the particularsolids content of the slurry employed, because an increase intemperature tends to lower the viscosity of the diluent. Thepolymerisation pressure is from 30 to 90 bars, most preferably about 41bars.

[0042] The minimum feed ratio of ethylene in the diluent, which ispreferably isobutane, is given by the solids concentration in thereactor, in the case without any settling:

[0043] Solids %=Weight of PE/(Weight of PE+Weight of diluent)

[0044] Without settling this approximates to:

[0045] p1 Solids %=Ethylene feed/(Ethylene feed+diluent feed)

[0046] Thus C2/diluent feed=1/((1/solids %)−1)

[0047] This is the minimum value for the C2/diluent feed ratio for agiven solids (purely no settling). The relationship between theC2/diluent feed ratio and the solids % is illustrated in FIG. 4. It maybe seen that the feed ratio generally increases with increase in thesolids % value, and it may be seen that a minimum value for theC2/diluent feed ratio for any given solids % value is provided. It isapparent that in order to get a high C2/diluent feed ratio, whichincreases reactor independence, it is necessary to maximise the solidscontent of the first reactor.

[0048] The maximum C2/diluent feed ratio in a system where settling isideally 100% completed depends on the nature of the fluff. The bulkdensity of the fluff in the settling leg will in fact control themaximum value for the C2/diluent feed ratio. The relationship betweenthe C2/diluent feed ratio and fluff bulk density is illustrated in FIG.5. This shows the maximum possible value of the C2/diluent feed ratiofor any given fluff bulk density, which depends on the fluffmorphological properties which are primarily controlled by the nature ofthe catalyst. Although the bulk density can reach as high as 0.5 g/cc insome cases, particularly with a metallocene catalyst, most commonly thebulk density is from 0.33 to 0.45 g/cc.

[0049] If there is a need to use a higher C2/diluent feed ratio, at afixed bulk density, one option is to use supercritical diluent in thereactor as described hereinbelow. With supercritical diluent, thesetemperature ranges given above for liquid diluent are about 10° C. to20° C. higher and reactor pressure from 37 to 100 bars, more preferablyfrom 50 to 90 bars, yet more preferably from 55 to 70 bars. Anotherconsequence of the use of supercritical diluent increases the C2/diluentfeed ratio, typically the C2/diluent ratio given in the FIGS. 4 and 5are twice as high, thus up to about 500%.

[0050] In addition, the fluff bulk density in the reactor also controlsthe maximum solids content at which the reactor can be operated. Therelationship between the maximum solids content and the bulk density isillustrated in FIG. 6.

[0051] In order to use a higher solids content in the first slurry loopreactor 4, the minimum operating pump power of the pump 16 is relativelyhigh and preferably the diameter D of the lower portion 22 of the firstslurry loop reactor 4 in which the propeller 18 is located is increasedto permit the pump more readily to circulate the higher solids contentslurry around the first slurry loop reactor 4. The power needed tocirculate a slurry having a given solids content around the reactordepends on the reactor diameter. If the reactor diameter is increased,less power is required, but that is at the expense of the coolingsurface area and consequently at the expense of reactor throughput. Fora reactor of particularly large size, the dissipation of such high powermay require the provision of multiple pumping sections or a pumpingsection with a large pump diameter, which allows dissipation of thepumping power in a larger volume of slurry, thereby avoiding thepotential of cavitation in the slurry. The provision of pumps in twodifferent pumping sections has the drawback that there is a higherprobability of bearing problems with two pumps and there is also arequirement for a long horizontal section between the two pumps or areactor with a larger number of vertical legs. The present inventor hasdetermined that for a variety of reactor sizes represented by a reactorvolume, there is a preferred internal diameter for the reactor, apreferred pump minimum operating power and a most preferred pump minimumoperating power. The corresponding values for three reactor sizes of19,70 and 100 m³ are summarised in Table 1.

[0052] In the second slurry loop reactor 6, the inventor has found thatthe degree of settling of the polyethylene resin fluff in the settlingleg 26 of the second slurry loop reactor 6 tends to increase with acorresponding increase in solid fraction and in the bulk density of thepolyethylene resin fluff in the second slurry loop reactor 6. For largescale production of bimodal polyethylene pipe resins, the second slurryloop reactor 6 may additionally be provided with a larger diametercirculating pump 32 having a construction similar to that of the pump 16for the first reactor 4 illustrated in FIG. 1 and continuous producetake-off through the settling leg 26, which would then be provided atthe bottom of the loop. This would provide the advantages of high plantthroughput and high catalyst productivity. The continuous product takeoff provides also the advantage of increasing plant reliability byeliminating a series of settling legs and corresponding valves.

[0053] The present inventor has found that when the solids fraction ofthe slurry circulating around the first slurry loop reactor 4 is above45 wt %, it is possible to employ, instead of periodic polyethylenefluff removal through the settling leg 10 illustrated in FIG. 1,continuous removal of the polyethylene fluff through a settling legprovided at the bottom of the reactor, which is thus absent a valve orwhich is just working with a open valve, this valve being closed forexample for reactor filling operations. The polyethylene resin fractionproduced in the first reactor is removed continuously from the settlingleg and continuously transferred to the second reactor 6.

[0054] Provided the solids concentration is sufficiently high, that isabove 45% of solids, this does not reduce significantly the reactorindependence. Such continuous removal and transfer of the firstpolyethylene resin fraction tends to reduce the possibility of thesettling leg being plugged by the resin and leakage problems. Moreover,it is believed by the inventor that the use of a continuous producttake-off as compared to a discontinuous produce take-off may tend toincrease the solids concentration in the first slurry loop reactor 4.

[0055] The inventor has found that the use of a high solidsconcentration in the first loop reactor leads not only to improvedmechanical properties of the polyethylene resin blend but also bymaximising the solids concentration in the first reactor, in particularby using a larger pump diameter and optionally continuous producttransfer of the polyethylene resin fraction to the second reactor at theend of a settling section, this leads to improved reactor throughput,improved product properties and improved catalyst productivity.

[0056] In accordance with preferred aspects of the invention, the use ofhigh solids content in the first reactor is achieved by providing asufficiently high pump power in the first reactor, which may be achievedby the provision of one or multiple pumping sections, and optionally byenlarging the section diameter in the pumping zones. Optionally, thesecond reactor should also be equipped with a large pump diameter. Mostpreferably, continuous product take-off is employed for transferring thepolyethylene resin fraction and catalyst from the first reactor to thesecond reactor. Optionally, a centrifuge or hydrocyclone may be employedin the transfer section between the two reactors, for reducing thetransfer of diluent and correspondingly comonomer or hydrogen, from thefirst reactor to the second reactor. Preferably, the second reactor alsoemploys continuous product take-off in order to maximize the solidsconcentration and reactor throughput of the second reactor.

[0057] In accordance with one preferred aspect of the invention, thediluent may be present under supercritical conditions. Thus the diluentis at a pressure greater than the critical pressure Pc and at atemperature greater than the critical temperature Tc. Under theseconditions, there is no thermodynamic transition between the gas phaseand the liquid phase and the homogeneous supercritical fluid has theproperties of a dense gas and a low density liquid. Either or both ofthe two loop reactors may be employed with supercritical diluent. Whenthe first loop reactor is operated under supercritical conditions, thepressure is typically from 37 to 100 bars, more preferably from 50 to 90bars, yet more preferably from 55 to 70 bars and the temperature istypically from 70 to 140° C., more preferably from 80 to 100° C. Undersupercritical conditions, as a result of faster settling and increasedpacking of the polyethylene fluff at the bottom of the settling leg,significantly less diluent is additionally removed with the polyethyleneresin fraction under supercritical conditions as compared to subcriticalconditions. This assists in enhancing the independence of the tworeactors.

[0058] The present invention will now be described in greater detailwith reference to the following non-limiting Examples.

EXAMPLE 1

[0059] The apparatus of FIG. 1 was employed to produce a bimodalpolyethylene resin in which in the first slurry loop reactor a highmolecular weight fraction was produced by copolymerisation of ethyleneand 1-hexene and in the second slurry loop reactor the low molecularweight fraction was produced by homopolymerisation of ethylene in thepresence of hydrogen. In the first reactor the temperature was 80 ° C.and the pressure was 42 bars and in the second reactor the temperaturewas 90° C. and the pressure was 41 bars. The catalyst comprised aZiegler-Natta catalyst activated with an alkyl aluminium. The firstreactor was fed with a feed of ethylene, comonomer and diluent, thediluent comprising isobutane, with the ethylene content of the feedbeing 71 wt % based on the weight of the diluent. The comonomerconcentration in the first reactor was adapted to reach the requireddensity, higher comonomer content resulting in lower density. Theremaining process parameters, comprising the solids content in the firstreactor (R1); the throughput through the reactors in tonnes per hour,the ratio of ethylene to diluent outputted from the first reactor (R1)(corresponding substantially (99%) to the the ratio of ethylene todiluent inputted to the first reactor (R1)); the ratio between 1-hexeneand ethylene in the second reactor (R2); and the ratio between thehydrogen and ethylene contents in the second reactor (R2), are specifiedin Table 2.

[0060] As compared to a typical PE100 pipe resin produced using aZiegler-Natta catalyst in two reactors in series, (a) the density of thefirst fraction which was half a point lower for the inventive example,and (b) the melt index in the first reactor and reactor proportions wereidentical, and of course the final resin melt index and density wereequal.

[0061] The resultant bimodal polyethylene resin fluff was extruded andstabilized with antioxidants and then the resultant resin was subjectedto a full notch creep test (FNCT) specified in ASTM F1473 in which thetime for failure was recorded for a circumferentially notched specimenhaving a 10 mm×10 mm cross section and a notch depth of 16% of thethickness, the specimen having been submitted to a net tensile strengthof 4 MPa at a temperature of 95° C. in a 2% solution of Arkopal fordetermining the slow crack growth resistance of the pipe resins . It maybe seen from Table 2 that the pipe resin exhibited a time to failureunder the FNCT test of at least 2000 hours.

[0062] Furthermore, the pipe resin was subjected to a Charpy impactenergy test to determine the resistance of the pipe resin to rapid crackpropagation in which the resin was tested under the procedures of ISO179/1A at a temperature of −10° C. and the Charpy impact strength isspecified in Table 2. Furthermore, the catalyst productivity isspecified in Table 2.

[0063] From Table 2 it may be seen that the ratio between hexene andethylene in the second reactor was low. This accordingly reduced thedegree of incorporation of hexene into the low molecular weighthomopolymer which was produced in the second reactor. Accordingly, thisincreased the density of the polyethylene resin fraction produced in thesecond reactor. As a consequence, the polyethylene resin fractionproduced in the first reactor may have an even lower density, since thefinal product density of a pipe resin is fixed by the technicalspecification. Having a lower density for the high molecular weightresin fraction produced in the first reactor allows a higher slow crackgrowth resistance (FNCT or NPT) to be achieved.

[0064] Referring to FIG. 7, there is shown the general relationshipbetween the slow crack growth resistance as determined by the notchedpipe test with the density of the polyethylene fluff in the firstreactor when the first reactor of the two serially connected reactors isemployed to produce the low density/high molecular weight resinfraction. It may be seen that the slow crack growth resistance tends toincrease with reduced density of the polyethylene fluff produced in thefirst reactor. This confirms that reactor independence is beneficial tothe slow crack growth resistance a property required by pipe resins, andalso film resins and blow moulding resins.

Comparative Example 1

[0065] In this Comparative Example, the apparatus of FIG. 1 was employedusing a lower solids content for the slurry in the first reactor and thecorresponding parameters and results are shown also in Table 2. It maybe seen that when the solids content is lower, in the ComparativeExample 25 wt % in the slurry of the first reactor, the hexene/ethyleneratio in the second reactor is increased as compared to the value forExample 1. This means that the density of the polyethylene resinfraction produced in the second reactor is increased and the density ofthe polyethylene resin fraction produced in the second reactor isdecreased. As a consequence, there is less separation between thereactors and less differential between the density of the two resinfractions. This means that the polyethylene resin fraction produced inthe first reactor is required to have a higher density than for Example1 with consequential decrease in the slow crack growth resistance, asconfirmed by the FNCT test for Comparative Example 1. Also, the Charpyimpact energy is reduced and the catalyst productivity is reduced inComparative Example 1 as compared to Example 1.

[0066] Furthermore, the throughput of reactors is also reduced.

EXAMPLE 2

[0067] In this Example a yet higher solids content in the first reactor,which was employed for producing a high molecular weight resin fractionin the first reactor and a low molecular weight resin fraction in thesecond reactor was employed, the solids content in the first reactorbeing 45 wt % based on the weight of the diluent. The correspondingprocess parameters are summarised in Table 3. It may be seen that withan even higher solids content in the first reactor, the throughput isyet further increased and the ethylene/isobutane ratio in the firstreactor is further increased. Most importantly, the 1-hexene/ethyleneratio in the second reactor is significantly reduced. In this Example,the first reactor was not provided with settling leg incorporating avalve but instead a product take-off from the loop was carried out fromthe first reactor. It may be seen that the low 1-hexene/ethylene ratioin the second reactor indicates a very high level of reactorindependence.

Comparative Example 2

[0068] When the apparatus employed for Example 2 was employed inComparative Example 2 at a lower solids content in the first reactor ofonly 17%, the throughput and the ethylene/isobutane ratio in the firstreactor were reduced. The results are shown also in Table 3. Mostimportantly, the 1-hexene/ethylene ratio in the second reactor wassubstantially increased as compared to Example 2, showing reducedindependence of the reactors at lower solids content. TABLE 1 Reactorinternal Preferred pump Most preferred Reactor diameter −cm minimumoperating minimum operating Size (m³) (inches) power (kW) pump power(kW) 10 56 (22) 200 220 70 61 (24) 330 400 100  61 (24) 450 530

[0069] TABLE 2 Comparative Example 1 Example 1 Solids wt % 32 25Throughput (T/h) 5 3.8 C2/iC4 out R1 (wt %) 0.71 0.58 [C6]/[C2−] in R2(wt %) 0.61 0.68 [H2]/[C2−] in R2 (vol %/wt %) 0.42 0.46 FNCT at 95° C.and 4 MPa (h) 2000 360 Charpy impact at −10° (J/m²) 15 14 Catalystproductivity (g/g) 9000 8000

[0070] TABLE 3 Comparative Example 2 Example 2 Solids wt % 45 17Throughput (kg/h) 10 8 C2/iC4 out R1 (wt %) 0.85 0.33 [C6]/[C2−] in R2(wt %) 0.067 0.175 [H2]/[C2−] in R2 (vol %/wt %) 21 15

1. A process for producing bimodal polyethylene resins in two reactorsin series, the process comprising producing a first polyethylene resinfraction in a first slurry loop reactor in a diluent in the presence ofa catalyst and producing a second polyethylene resin fraction in asecond slurry loop reactor, serially connected to the first reactor, inthe diluent in the presence of the catalyst, the first polyethyleneresin fraction being passed from the first reactor to the second reactortogether with the catalyst, one of the first and second reactorsproducing a resin fraction of higher molecular than the resin fractionproduced by the other of the first and second reactors, characterised inthat the first reactor is fed with a feed of ethylene and diluent havingan ethylene content of at least 70 wt % based on the weight of thediluent and in that in the first reactor the slurry of polyethylene inthe diluent has a solids content of at least 30 wt % based on the weightof the diluent.
 2. A process according to claim 1 wherein the catalystis a Ziegler-Natta catalyst and the slurry in the first reactor has asolids content of from 30 to 60 wt % based on the weight of the diluent.3. A process according to claim 1 or claim 2 wherein the catalyst is aZiegler-Natta catalyst and the feed of ethylene and diluent for thefirst reactor has an ethylene content of from 70 to 250 wt % based onthe weight of the diluent.
 4. A process according to claim 1 wherein thecatalyst is a metallocene catalyst and the slurry in the first reactorhas a solids content of from 35 to 60 wt % based on the weight of thediluent.
 5. A process according to claim 1 or claim 4 wherein thecatalyst is a metallocene catalyst and the feed of ethylene and diluentfor the first reactor has an ethylene content of from 70 to 250 wt %based on the weight of the diluent.
 6. A process according to anyforegoing claim wherein the slurry of polyethylene and diluent in thefirst reactor has a solids content of at least 45 wt % and the firstpolyethylene resin fraction is continuously removed from the firstreactor.
 7. A process according to any foregoing claim wherein therelatively high molecular weight fraction is polymerised in the firstreactor by copolymerisation of ethylene and a comonomer comprising atleast one α-olefin having from 3 to 12 carbon atoms.
 8. A processaccording to any foregoing claim wherein the relatively low molecularweight polyethylene resin fraction is produced in the second reactor byhomopolymerisation of ethylene in the presence of hydrogen.
 9. A processaccording to any foregoing claim wherein the diluent is undersupercritical conditions at least in the first reactor.
 10. A processaccording to any foregoing claim wherein the polyethylene resin iscontinuously transferred from the first to the second reactor.
 11. Aprocess according to any foregoing claim wherein the polyethylene resinis continuously removed from the second reactor.
 12. A process accordingto any foregoing claim wherein the relatively low molecular weightpolyethylene resin fraction is produced in the first reactor byhomopolymerisation of ethylene in the presence of hydrogen.
 13. Aprocess according to any foregoing claim wherein the relatively highmolecular weight fraction is polymerised in the second reactor bycopolymerisation of ethylene and a comonomer comprising at least oneα-olefin having from 3 to 12 carbon atoms.